Methods and systems for cryogenic cooling

ABSTRACT

Methods and systems are provided for cooling an object with a cryogen having a critical point defined by a critical-point pressure and a critical-point temperature. A pressure of the cryogen is raised above a pressure value determined to provide the cryogen at a reduced molar volume that prevents vapor lock. Thereafter, the cryogen is placed in thermal communication with the object to increase a temperature of the cryogen along a thermodynamic path that maintains the pressure greater than the critical-point pressure for a duration that the cryogen and object are in thermal communication.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a Division of U.S. patent application Ser. No.10/952,531 entitled METHODS AND SYSTEMS FOR CRYOGENIC COOLING which is acontinuation-in-part of U.S. patent application Ser. No. 10/757,769,(U.S. Pat. No. 7,083,612 issued Aug. 1, 2006) entitled “CRYOTHERAPYSYSTEM,” filed Jan. 14, 2004 by Peter J. Littrup et al., which is anonprovisional claiming the benefit of the filing date of U.S. Prov.Appl. No. 60/440,662, entitled “CRYOSURGICAL SYSTEM,” filed Jan. 15,2003 by Peter Littrup et al. This application is also acontinuation-in-part of U.S. patent application Ser. No. 10/757,768,entitled “CRYOTHERAPY PROBE,” filed Jan. 14, 2004 by Peter J. Littrup etal., which is a nonprovisional claiming the benefit of the filing dateof U.S. Prov. Pat. Appl. No. 60/440,541, entitled “CRYOSURGICAL PROBE,”filed Jan. 15, 2003 by Peter Littrup et al. The entire disclosures U.S.patent application Ser. No. 10/952,531; U.S. patent application Ser. No.10/757,769, U.S. Pat. No. 7,083,612; U.S. patent application Ser. No.10/757,768; U.S. Prov. Appl. No. 60/440,662; and U.S. Prov. Pat. Appl.No. 60/440,541 are incorporated herein by reference for all purposes,including the appendices.

BACKGROUND OF THE INVENTION

This application relates to methods and systems for cryogenic cooling.“Cryogenic cooling” refers generally to processes that use liquefiedgases, i.e. “cryogens,” in providing the cooling, which may take theform of freezing or simply chilling a system or material.

There are numerous applications, both medical and nonmedical, in whichit is desirable to provide effective cooling. Any cooling process may beconsidered as involving one or more of four basic processes that resultin removal of a heat load: evaporation, conduction, radiation, andconvection. One challenge that is presented in cryogenic coolingtechniques results from the process of evaporation, and may beunderstood by considering cooling within a small channel. The process ofevaporation of a liquefied gas results in enormous expansion as theliquid converts to a gas; the volume expansion is on the order of afactor of 200. In a small-diameter system, this degree of expansionconsistently results in a phenomenon known in the art as “vapor lock.”The phenomenon is exemplified by the flow of a cryogen in athin-diameter tube, such as is commonly provided in a cryoprobe. Arelatively massive volume of expanding gas that forms ahead of itimpedes the flow of the liquid cryogen. Traditional techniques that havebeen used to avoid vapor lock have included restrictions on the diameterof the tube, requiring that it be sufficiently large to accommodate theevaporative effects that lead to vapor lock. Other complex cryoprobe andtubing configurations have been used to “vent” N₂ gas as it formed alongtransport tubing. These designs also contributed to limiting the costefficacy and probe diameter.

There is accordingly a general need in the art for improved methods andsystems for providing cryogenic cooling.

BRIEF SUMMARY OF THE INVENTION

Embodiments of the invention thus provide methods and systems forcooling an object. The methods and systems provide changes inpressure/temperature characteristics that avoid the occurrence of vaporlock by following a thermodynamic path that never crosses a liquid-gasphase line of the cryogen. In a number of embodiments, this is achievedby keeping the pressure of the cryogen near or above its critical-pointpressure. In many embodiments, the cryogen comprises nitrogen (N₂),although other cryogens may be used in other embodiments.

In a first set of embodiments, a method is provided for cooling anobject with a cryogen having a critical point defined by acritical-point pressure and a critical-point temperature. A pressure ofthe cryogen is raised above its critical-point pressure. Thereafter, thecryogen is placed n thermal communication with the object to increase atemperature of the cryogen along a thermodynamic path that maintains thepressure greater than the critical-point pressure for a duration thatthe cryogen and object are in thermal communication.

In some instances, the thermodynamic path may increase the temperatureof the cryogen to an ambient temperature. The pressure of the cryogenmay be reduced to an ambient pressure after removing the cryogen fromthermal communication with the object. In some embodiments, afterraising the pressure of the cryogen above its critical-point temperatureand prior to placing the cryogen in thermal communication with theobject, the temperature of the cryogen is reduced without decreasing thepressure of the cryogen below its critical-point pressure. For example,the temperature of the cryogen may be reduced by placing the cryogen inthermal communication with a second liquid cryogen having a temperaturelower than the temperature of the cryogen. The cryogen and secondcryogen may be chemically identical, and the second liquid cryogen maybe substantially at ambient pressure. In one embodiment, the pressure ofthe cryogen is substantially constant while reducing the temperature ofthe cryogen.

When the pressure of the cryogen is raised above its critical-pointpressure, the pressure of the cryogen may be raised to near orapproximately at its critical-point pressure. Also, when the pressure ofthe cryogen is raised above its critical-point pressure, the cryogen maybe provided at approximately its critical-point temperature. In oneembodiment, the cryogen is provided at a temperature within ±10% of itscritical-point temperature. The pressure of the cryogen may be raisedabove its critical-point pressure in one embodiment by placing thecryogen within a thermally insulated tank and applying heat within thetank at least until a predetermined pressure within the tank is reached.

In a second set of embodiments, another method is provided for coolingan object with a cryogen having a critical point defined by acritical-point pressure and a critical-point temperature. A pressure ofthe cryogen is raised to between 0.8 and 1.2 of its critical pointpressure and the cryogen is provided at a temperature within ±10% of itscritical-point temperature. The temperature of the cryogen is thereafterreduced without decreasing the pressure below 0.8 times itscritical-point pressure. Thereafter, the cryogen is placed in thermalcommunication with the object to increase the temperature of the cryogento an ambient temperature along a thermodynamic path that maintains apressure greater than 0.8 times the critical-point pressure for aduration that the cryogen and object are in thermal communication. Thecryogen is subsequently removed from thermal communication with theobject and the pressure of the cryogen is reduced to an ambientpressure.

In a third set of embodiments, a further method is provided for coolingan object with a cryogen having a critical point defined by acritical-point pressure and a critical-point temperature. A pressure ofthe cryogen is raised to near its critical-point pressure. Thereafter,the cryogen is placed in thermal communication with the object toincrease a temperature of the cryogen along a thermodynamic path thatmaintains the pressure near the critical-point pressure for a durationthat the cryogen and object are in thermal communication. The pressuremay be raised to near the critical-point pressure by raising thepressure of the cryogen to greater than a pressure value determined toprovide the cryogen at a reduced molar volume that prevents vapor lock,with the thermodynamic path maintaining the pressure above thedetermined pressure value. In some instances, the determined pressurevalue is between 0.8 and 1.2 times the critical-point pressure.

In a fourth set of embodiments, a system is provided for cooling anobject with a cryogen having a critical point defined by acritical-point pressure and a critical-point temperature. A cryogengenerator is adapted to increase a pressure of the cryogen. A valve isprovided at an outlet of the cryogen generator and adapted to release aflow of the cryogen when the pressure of the cryogen exceeds apredetermined pressure within the cryogen generator. The predeterminedpressure is greater than a pressure value determined to provide thecryogen at a reduced molar volume that prevents vapor lock. A cryogenicapplication device is also provided and adapted to be brought intothermal communication with the object. A conduit connects the valve withthe cryogenic application device for transporting the cryogen from thevalve to the cryogenic application device. A flow controller regulatesflow of cryogen through the conduit and cryogenic application device.The cryogen increases in temperature when the application device is inthermal communication with the object along a thermodynamic path thatmaintains the pressure of the cryogen above the predetermined pressurefor a duration that the application device and cryogen are in thermalcommunication.

The cryogen generator may comprise a thermally insulated tank having aninterior volume for holding the cryogen and a heating element forapplying heat within the interior volume. In one embodiment, the heatingelement comprises a resistive heating element. A bath of a second liquidcryogen may surround a portion of the conduit between the valve and thecryogenic application device. In one embodiment, the bath of the secondliquid cryogen is at substantially ambient pressure. In anotherembodiment, the cryogen and second cryogen are chemically identical.

In certain embodiments, the cryogen generator comprises a plurality ofcryogen generators and the valve comprises a plurality of valves. Eachof the plurality of cryogen generators is adapted to increase thepressure of the cryogen. Each of the plurality of valves is provided atan outlet of one of the plurality of cryogen generators and configuredto release a flow of the cryogen when the pressure of the cryogenexceeds a predetermined pressure within the one of the plurality ofcryogen generators. The conduit is configured to provide a selectiveconnection between one of the plurality of valves and the cryogenicapplication device. In some such embodiments, the system furthercomprises a liquid bath of the cryogen, with the plurality of cryogengenerators disposed within the liquid bath. In addition, the system mayfurther comprise a plurality of heat exchangers, each of which isdisposed within the liquid bath of the cryogen and in fluidcommunication between a respective one of the plurality of cryogengenerators and the conduit. Each of the plurality of cryogen generatorsmay additionally comprise an inlet to receive cryogen from the liquidbath of the cryogen. The cryogenic application device may comprise adetachable spray control nozzle, which in some instances may comprise avent adapted to release cryogen reflected during use of the cryogenicapplication device. In one embodiment, the system further comprises athermometry device adapted to measure a temperature of the object.

In a fifth set of embodiments, a method is provided for cooling anobject with a cryogen having a critical point defined by acritical-point pressure and a critical-point temperature. A pressure ofthe cryogen is raised in a first cryogen generator above a pressurevalue determined to provide the cryogen at a reduced molar volume thatprevents vapor lock. Thereafter, the cryogen is flowed from the firstcryogen generator to be in thermal communication with the object and tohave a pressure greater than the critical-point pressure while thecryogen and object are in thermal communication. Thereafter, a pressureof the cryogen in a second cryogen generator is raised above thedetermined pressure value. Thereafter, the cryogen from the secondcryogen generator is flowed to be in thermal communication with theobject and to have a pressure greater than the determined pressure valuewhile the cryogen and object are in thermal communication. Thereafter,cryogen in the first cryogen generator is replenished.

In some such embodiments, when cryogen is flowed from the first orsecond cryogen generator, it is flowed through a conduit in thermalcommunication with a liquid bath to reduce a temperature of the cryogen.In one embodiment, the cryogen and liquid cryogen bath are chemicallyidentical, with cryogen in the first cryogen generator being replenishedfrom the liquid cryogen bath. When the pressure of the cryogen is raisedabove the determined pressure value in the first and second cryogengenerators, it may be raised to near its critical-point pressure. In oneembodiment, applying heat within a thermally insulated tank at leastuntil a predetermined pressure within the thermally insulated tank isreached raises the pressure.

BRIEF DESCRIPTION OF THE DRAWINGS

A further understanding of the nature and advantages of the presentinvention may be realized by reference to the remaining portions of thespecification and the drawings wherein like reference numerals are usedthroughout the several drawings to refer to similar components. In someinstances, a sub-label is associated with a reference numeral andfollows a hyphen to denote one of multiple similar components. Whenreference is made to a reference numeral without specification to anexisting sub-label, it is intended to refer to all such multiple similarcomponents.

FIG. 1A illustrates a typical cryogen phase diagram;

FIG. 1B provides an illustration of how to determine a minimum operatingpressure for a cryogenic probe;

FIG. 1C uses a cryogen phase diagram to illustrate the occurrence ofvapor lock with simple-flow cryogen cooling;

FIG. 1D uses a cryogen phase diagram to illustrate a cooling cycle usedin Joule-Thomson cooling to avoid the occurrence of vapor lock;

FIG. 2A is a schematic illustration of a cryogenic cooling systemaccording to an embodiment of the invention;

FIG. 2B uses a cryogen phase diagram to illustrate a method forcryogenic cooling in an embodiment of the invention;

FIG. 3 provides a flow diagram that summarizes aspects of the coolingmethod of FIG. 2A;

FIG. 4 is a schematic illustration of a cryogenic cooling systemaccording to another embodiment of the invention;

FIG. 5 is a schematic illustration of a cryogenic cooling systemaccording to a further embodiment of the invention;

FIG. 6 is a photograph of an embodiment of a cryogenic system configuredas a self-contained handheld device;

FIG. 7A shows another handheld embodiment that allows interchangeabletips to direct the near-critical nitrogen as a pinpoint spray or as amore evenly dispersed circular coverage for areas of various diameter;

FIG. 7B illustrates a combined surface treatment of o tumor with a conedtip in combination with an interstitial vented needle, as well as deepermonitoring with a multi-point resistive thermometry array; and

FIG. 8 provides a graphical comparison of cooling power for differentcryogenic cooling processes.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the invention make use of thermodynamic processes usingcryogens that provide cooling without encountering the phenomenon ofvapor lock. Various other benefits and advantages provided in differentembodiments of the invention are apparent from the followingdescription.

This application uses phase diagrams to illustrate and compare variousthermodynamic processes. Such phase diagrams are well known in the artand an example typical for a cryogen is provided in FIG. 1A. The axes ofthe diagram correspond to pressure P and temperature T, and includes aphase line 102 that delineates the locus of all (P, T) points whereliquid and gas coexist. For (P, T) values to the left of the phase line102, the cryogen is in a liquid state, generally achieved with higherpressures and lower temperatures, while (P, T) values to the right ofthe phase line 102 define regions where the cryogen is in a gaseousstate, generally achieved with lower pressures and higher temperatures.The phase line 102 ends abruptly in a single point known as the criticalpoint 104. In the case of nitrogen N₂, the critical point is atP_(c)=33.94 bar and T_(c)=−147.15° C.

When a fluid has both liquid and gas phases present during a gradualincrease in pressure, the system moves up along the liquid-gas phaseline 102. In the case of N₂, the liquid at low pressures is up to twohundred times more dense than the gas phase. A continual increase inpressure causes the density of the liquid to decrease and the density ofthe gas phase to increase, until they are exactly equal only at thecritical point 104. The distinction between liquid and gas disappears atthe critical point 104. The blockage of forward flow by gas expandingahead of the liquid cryogen is thus avoided by conditions surroundingthe critical point, defined herein as “near-critical conditions.”Factors that allow greater departure from the critical point whilemaintaining a functional flow include greater speed of cryogen flow,larger diameter of the flow lumen and lower heat load upon the thermalexchanger, or cryoprobe tip.

As the critical point is approached from below, the vapor phase densityincreases and the liquid phase density decreases until right at thecritical point, where the densities of these two phases are exactlyequal. Above the critical point, the distinction of liquid and vaporphases vanishes, leaving only a single, supercritical phase. All gasesobey quite well the following van der Waals equation of state:

$\begin{matrix}{{{\left( {p + \frac{3}{v^{2}}} \right)\left( {{3\; v} - 1} \right)} = {8t}},} & \left\lbrack {{Eq}.\mspace{14mu} 1} \right\rbrack\end{matrix}$where p≡P/P_(c), v≡V/V_(c), and t=T/T_(c), and P_(c), V_(c), and T_(c)are the critical pressure, critical molar volume, and the criticaltemperature respectively. The variables v, p, and t are often referredto as the “reduced molar volume,” the “reduced pressure,” and the“reduced temperature,” respectively. Hence, any two substances with thesame values of p, v, and t are in the same thermodynamic state of fluidnear its critical point. Eq. 1 is thus referred to as embodying the “Lawof Corresponding States.” This is described more fully in H. E. Stanley,Introduction to Phase Transitions and Critical Phenomena (Oxford SciencePublications, 1971), the entire disclosure of which is incorporatedherein by reference for all purposes. Rearranging Eq. 1 provides thefollowing expression for v as a function of p and t:pv ³−(p+8t)v ²+9v−3=0.  [Eq. 2]The reduced molar volume of the fluid v may thus be thought of as beingan exact function of only the reduced pressure t and the reducedpressure p.

Typically, in embodiments of the invention, the reduced pressure p isfixed at a constant value of approximately one, and hence at a fixedphysical pressure near the critical pressure, while the reducedtemperature t varies with the heat load applied to the needle. If thereduced pressure p is a constant set by the engineering of the system,then the reduced molar volume v is an exact function of the reducedtemperature t. In embodiments of the invention, the needle's operatingpressure p may be adjusted so that over the course of variations in thetemperature t of the needle, v is maintained below some maximum value atwhich the vapor lock condition will result. It is generally desirable tomaintain p at the lowest value at which this is true since boosting thepressure to achieve higher values of p may involve use of a more complexand more expensive compressor, resulting in more expensive procurementand maintenance of the entire needle support system and lower overallwall plug efficiency. As used herein, “wall plug efficiency” refers tothe total cooling power of a needle divided by the power obtained from aline to operate the system.

The conditions that need to be placed on v depend in a complex andnon-analytic way on the volume flow rate dV/dt, the heat capacity of theliquid and vapor phases, and the transport properties such as thethermal conductivity, viscosity, etc., in both the liquid and the vapor.This exact relationship cannot be derived in closed form algebraically,but may be determined numerically by integrating the model equationsthat describe mass and heat transport within the needle. Conceptually,vapor lock occurs when the rate of heating of the needle produces thevapor phase, and when the cooling power of this vapor phase, which isproportional to the flow rate of the vapor times its heat capacitydivided by its molar volume, is not able to keep up with the rate ofheating to the needle. When this occurs, more and more of the vaporphase is formed in order to absorb the excess heat through theconversion of the liquid phase to vapor in the cryogen flow. Thiscreates a runaway condition where the liquid converts into vapor phaseto fill the needle, and effectively all cryogen flow stops due to thelarge pressure that results in this vapor phase as the heat flow intothe needle increases its temperature and pressure rapidly. Thiscondition is called “vapor lock.” Since the liquid and vapor phases areidentical in their molar volume, and hence cooling power at the criticalpoint, the cooling system at or above the critical point can never vaporlock. But for conditions slightly below the critical below the criticalpoint, the needle may avoid vapor lock as well. A relationship between aminimum acceptable molar volume, corresponding to the minimum acceptablegas phase density, and dimensions of the needle, flow rate, andthermophysical properties of gas and liquid phases is a consequence of amanifestly complex nonlinear system. A determination of how large v maybe, and hence how small p may be, to reliably avoid vapor lock may bedetermined experimentally, as illustrated with the data shown in FIG.1B.

FIG. 1B displays how a minimum operating pressure P, and hence theminimum reduced pressure p, is determined experimentally. The uppercurve in the top panel shows the pressure of nitrogen in the needle andthe bottom curve in the top panel shows the resulting mass flow ratethrough the probe, displayed in units of standard liters per secondthrough the needle. The bottom panel shows the needle tip temperature atthe same times as the top plot. A heat load of 6.6 W was applied to theneedle tip while these data were taken. For example, at an operatingpressure of 12.6 bar and 22 bar a vapor-lock condition occurred at thislevel of heat load and flow rate, as evidenced by the failure of theneedle tip temperature to recover its low temperature value when theflow was momentarily interrupted and then resumed. But at 28.5 bar ofpressure, the tip temperature recovered its low temperature valuereliably following a flow interruption. The downwards trend in the massflow rate through the needle is indicative of being very close, yetslightly below the lowest acceptable pressure for reliable, continuousoperation without vapor lock. These data suggest that about 29 bars ofpressure is the lowest acceptable operating pressure in thisillustrative embodiment. Thus, for this embodiment, in which avacuum-jacketed needle with 22-cm long capillaries of 0.020-cm diameterfor the inflow capillary and 0.030-cm diameter for the outflowcapillary, under this heat load and flow rate, 29 bar is a typicalminimum operating pressure. This corresponds to a minimum p=29 bar/33.9bar=0.85. Hence, in this illustrative embodiment, “near critical”corresponds to a pressure no less than 85% of the critical pressure.

More generally, references herein to a “near-critical” pressure areintended to refer to a pressure that exceeds a minimum pressuredetermined to meet the conditions described above. In particular, oncethe minimum value of p has been determined, such as with theexperimental procedure described, the “near-critical” pressure rangeincludes all values of p=P/P_(c) that are at or above the determinedminimum pressure. While any supercritical pressure having p>1 isgenerally acceptable to avoid vapor lock, the inventors have discoveredthat in practice the minimum value of p that may be used withoutcreating vapor-lock conditions may be lower, and use of such a lowervalue may advantageously improve system efficiency and simplicity.

The occurrence of vapor lock in a simple-flow cryogen cooling system maybe understood with reference to FIG. 1C, which for exemplary purposesshows the phase diagram for N₂, with liquid-gas phase line 106terminating at critical point 108. The simple-flow cooling proceeds bycompressing the liquid cryogen and forcing it to flow through acryoprobe. Some pre-cooling may be used to force liquid-phase cryogenthrough an inlet 110 of the cryoprobe from the indicated point on thephase diagram to the region where the cryogen evaporates to provideevaporative cooling. The thermodynamic path 116 taken by the cryogen asit is forced from the inlet 110 to a vent 114 intersects the liquid-gasphase line 106 at point 112, where the evaporation occurs. Because theevaporation occurs at a point along the liquid-gas phase line 106 wellbelow the critical point 108, there is a dramatic expansion of thevolume of the flow stream as the much denser liquid evaporates into itsgaseous phase, leading to the occurrence of vapor lock.

An alternative cryogen cooling technique that avoids vapor lock at theexpense of a number of complexities exploits the Joule-Thomson effect.When a gas is compressed, there is a reduction in its enthalpy, the sizeof the reduction varying with the pressure. When the gas is thenexpanded through a small port (referred to as a “JT port” or “throttle”)to a lower pressure, there is a reduction in temperature, with theresultant cooling being a function of the decrease in enthalpy duringcompression. With a heat exchanger provided between the compressor andexpansion valve, progressively lower temperatures may be reached. Insome instances, Joule-Thomson cooling uses cheaper gases like CO₂ orN₂O, although lower temperatures can be achieved with argon (Ar). Theremay be higher risks associated with Ar in addition to its higher cost,but both of these may be justified in some applications because of therapid initiation and termination of freezing that may be provided.

Joule-Thomson cooling processes thus use a completely different coolingcycle than is used for simple-flow cryogen cooling, as illustrated withthe phase diagram of FIG. 1D. The cooling cycle is shown superimposed onthe N₂ phase diagram as a specific example, with the liquid-gas phaseline 122 for N₂ terminating at its critical point 128. Nitrogen isinitially provided at very high pressures at normal ambient (room)temperature at point 130 on the phase diagram. The pressure is typicallyabout 400 bar, i.e. greater than ten times the pressure at the criticalpoint 128. The N₂ flows within a cryoprobe along thermodynamic path 124until it reaches the JT expansion port at point 132 on the phasediagram. The N₂ expands abruptly at the JT port, flowing in a JT jet 142downwards in the phase diagram as its pressure decreases rapidly. Therapid expansion causes the N₂ downstream in the jet 142 to partiallyliquefy so that following the expansion at the JT jet 142, the liquefiedN₂ is in thermal equilibrium with its gaseous phase. The nitrogen isthus at point 134 in the phase diagram, i.e. on the liquid-gas phaseline 106 slightly above ambient pressure, and therefore well below thecritical point 128. The nitrogen is heated on a return gas streamfollowing thermodynamic path 126 where it may be used for cooling, andis subsequently exhausted to ambient conditions through a vent 140,perhaps on the way back to a controlling console. It is notable thatJoule-Thomson cooling never approaches the critical point of theliquid-gas system, and that it uses predominantly evaporative-flowcooling.

The flow of the cooled gas in Joule-Thomson cooling is typicallyprovided back along a side of the inlet high-pressure feed line. Thiscounter-flow of the low-pressure return gas advantageously cools theincoming high-pressure gas before expansion. The effect of this heatexchanger 144 between the gas streams is evident in the phase diagramsince the pressure along the inlet line to the JT port (thermodynamicpath 124) falls due to its flow impedance as the stream of high-pressuregas is cooled by the counter-flow heat exchanger. Similarly, thepressure of the return stream (thermodynamic path 126) falls slightly asthe cold, low-pressure nitrogen cools the incoming stream at highpressure through the counter-flow heat exchanger 144. The effects of thecounter-flow heat exchanger 144 are beneficial in improving theefficiency the Joule-Thomson cooling, but limits to this efficiencyresult from trying to make the cryoprobe needle smaller in diameter. Asthe cryoprobe needle becomes smaller, the return-gas-flow velocitybecomes larger, eventually reaching the speed of sound for typicalvolume flow rates and probe designs in probes having a diameter of about1.5 mm. The Joule-Thomson cooling process continues to lose efficiencyas the probe is miniaturized further, to the point where no more coolingpower can be generated. Probes with diameters <1.2 mm are therebyseverely limited by the physics of their operation to the point wherethey would have minimal cooling capacity, even if they could be reliablyconstructed at a reasonable cost. The cost of Joule-Thomson probeconstruction increases rapidly as the probe diameter is reduced,primarily because of the fabrication and assembly costs associated withthe counter-flow heat exchanger.

Embodiments of the invention avoid the occurrence of vapor lock andpermit decreased probe sizes by operating in cryogenpressure-temperature regimes that avoid any crossing of the liquid-gasphase line. In particular embodiments, cryogenic cooling is achieved byoperating near the critical point for the cryogen. When operating inthis region, heat flows into the near-critical cryogen from thesurrounding environment since the critical-point temperature (e.g.,−147° C. in the case of N₂) is much colder that the surroundingenvironment. This heat is removed by the flow of the near criticalcryogen through the tip of a cryoprobe, even though there is no latentheat of evaporation to assist with the cooling process. While the scopeof the invention is intended to include operation in any regime having apressure greater than the critical-point pressure, the coolingefficiency tends to decrease as the pressure is increased above thecritical pressure. This is a consequence of increasing energyrequirements needed to achieve flow at higher operating pressures.

FIG. 2A provides a schematic illustration of a structural arrangementfor a cryogenic system in one embodiment, and FIG. 2B provides a phasediagram that illustrates a thermodynamic path taken by the cryogen whenthe system of FIG. 2A is operated. The circled numerical identifiers inthe two figures correspond so that a physical position is indicated inFIG. 2A where operating points identified along the thermodynamic pathare achieved. The following description thus sometimes makessimultaneous reference to both the structural drawing of FIG. 2A and tothe phase diagram of FIG. 2B in describing physical and thermodynamicaspects of the cooling flow. For purposes of illustration, both FIGS. 2Aand 2B make specific reference to a nitrogen cryogen, but this is notintended to be limiting. The invention may more generally be used withany suitable cryogen, as will be understood by those of skill in theart; merely by way of example, alternative cryogens that may be usedinclude argon, helium, hydrogen, and oxygen. In FIG. 2B, the liquid-gasphase line is identified with reference label 256 and the thermodynamicpath followed by the cryogen is identified with reference label 258.

A cryogenic generator 246 is used to supply the cryogen at a pressurethat exceeds the critical-point pressure P_(c) for the cryogen at itsoutlet, referenced in FIGS. 2A and 2B by label ◯. The cooling cycle maygenerally begin at any point in the phase diagram having a pressureabove or slightly below P_(c), although it is advantageous for thepressure to be near the critical-point pressure P_(c), The coolingefficiency of the process described herein is generally greater when theinitial pressure is near the critical-point pressure P_(c) so that athigher pressures there may be increased energy requirements to achievethe desired flow. Thus, embodiments may sometimes incorporate varioushigher upper boundary pressure but generally begin near the criticalpoint, such as between 0.8 and 1.2 times P_(c), and in one embodiment atabout 0.85 times P_(c), Different embodiments may have the initialcryogen pressure greater than about 0.8 times its critical-pointpressure, but less than 5.0 times P_(c), less than 2.0 times P_(c), lessthan 1.5 times P_(c), less than 1.2 times P_(c), less than 1.1 timesP_(c), less than 1.01 times P_(c), or less than 1.001 times P_(c), Also,in several embodiments, it is advantageous for the initial temperatureof the cryogen to be at or near the critical-point temperature T_(c) sothat in different embodiments the initial temperature is T_(c)±50° C.,is T_(c)±25° C., is T_(c)±10° C., is T_(c)±5° C., is T_(c)±1° C., isT_(c)±0.1° C., or is substantially equal to T_(c).

The cryogen is flowed through a tube, at least part of which issurrounded by a reservoir 240 of the cryogen in a liquid state, reducingits temperature without substantially changing its pressure. In FIG. 2A,reservoir is shown as liquid N₂, with a heat exchanger 242 providedwithin the reservoir 240 to extract heat from the flowing cryogen.Outside the reservoir 240, thermal insulation 220 may be provided aroundthe tube to prevent unwanted warming of the cryogen as it is flowed fromthe cryogen generator 246. At point ◯, after being cooled by beingbrought into thermal contact with the liquid cryogen, the cryogen has alower temperature but is at substantially the initial pressure. In someinstances, there may be a pressure change, as is indicated in FIG. 2B inthe form of a slight pressure decrease, provided that the pressure doesnot drop substantially below the critical-point pressure P_(c), i.e.does not drop below the determined minimum pressure. In the exampleshown in FIG. 2B, the temperature drop as a result of flowing throughthe liquid cryogen is about 47° C.

The cryogen is then provided to a device for use in cryogenicapplications. In the exemplary embodiment shown in FIG. 2A, the cryogenis provided to an inlet 236 of a cryoprobe 224, such as may be used inmedical cryogenic applications, but this is not a requirement. At thepoint when the cryogen is provided to such a device, indicated by label◯ in FIGS. 2A and 2B, there may be a slight change in pressure and/ortemperature of the cryogen as it moves through an interface with thedevice, i.e. such as when it is provided from the tube to the cryoprobeinlet 236 in FIG. 2A. Such changes may typically show a slight increasein temperature and a slight decrease in pressure. Provided the cryogenpressure remains above the determined minimum pressure (and associatedconditions), slight increases in temperature do not significantly affectperformance because the cryogen simply moves back towards the criticalpoint without encountering the liquid-gas phase line 256, therebyavoiding vapor lock.

Thermal insulation along the shaft of the cryotherapy needles, and alongthe support system that delivers near-critical freeze capability tothese needles, may use a vacuum of better than one part per million ofatmospheric pressure. Such a vacuum may not be achieved by conventionaltwo-stage roughing pumps alone. The percutaneous cryotherapy system inan embodiment thus incorporates a simplified method of absorptionpumping rather than using expensive and maintenance-intensivehigh-vacuum pumps, such as diffusion pumps or turbomolecular pumps. Thismay be done on an internal system reservoir of charcoal, as well asbeing built into each individual disposable probe.

Embodiments of the system incorporate a method of absorption pumping inwhich the liquid nitrogen bath that is used to sub-cool the stream ofincoming nitrogen near its critical point is also used to cool a smallvolume of clean charcoal. The vast surface area of the charcoal permitsit to absorb most residual gas atoms, thus lowering the ambient pressurewithin its volume to well below the vacuum that is used to thermallyinsulate the needle shaft and the associated support hardware. Thisvolume that contains the cold charcoal is attached throughsmall-diameter tubing to the space that insulates the near-criticalcryogen flow to the needles. Depending upon the system designrequirements for each clinical use, the charcoal may be incorporatedinto the cooling reservoir of liquid cryogen 240 seen in FIG. 2A, orbecome part of the cryoprobe 224, near the connection of the extensionhose near the inlet 236. Attachments may be made through a thermalcontraction bayonet mount to the vacuum space between the outer shaft ofthe vacuum-jacketed needles and the internal capillaries that carry thenear-critical cryogen, and which is thermally insulated from thesurrounding tissue. In this manner, the scalability of the systemextends from simple design constructions, whereby the charcoal-vacuumconcept may be incorporated into smaller reservoirs where it may be moreconvenient to draw the vacuum. Alternatively, it may be desirable formultiple-probe systems to individually incorporate small charcoalpackages into each cryoprobe near the junction of the extensionclose/cryoprobe with the machine interface 236, such that each hose andcryoprobe draws its own vacuum, thereby further reducing constructioncosts.

Flow of the cryogen from the cryogen generator 246 through the cryoprobe224 or other device is controlled in the illustrated embodiment with anassembly that includes a crack valve 216, a flow impedance, and a flowcontroller. The cryoprobe 224 itself may comprise a vacuum jacket 232along its length and may have a cold tip 228 that is used for thecryogenic applications. Unlike a Joule-Thomson probe, where the pressureof the working cryogen changes significantly at the probe tip, theseembodiments of the invention provide relatively little change inpressure throughout the probe. Thus, at point ◯, the temperature of thecryogen has increased approximately to ambient temperature, but thepressure remains elevated. By maintaining the pressure above thecritical-point pressure P_(c) throughout the process, the liquid-gasphase line 256 is never encountered along the thermodynamic path 258 andvapor lock is thereby avoided. The cryogen pressure returns to ambientpressure at point ◯ before passing through the flow controller 208,which is typically located well away from the cryoprobe 224. The cryogenmay then be vented through vent 204 at substantially ambient conditions.

A method for cooling in one embodiment in which the cryogen follows thethermodynamic path shown in FIG. 2B is illustrated with the flow diagramof FIG. 3. At block 310, the cryogen is generated with a pressure thatexceeds the critical-point pressure and is near the critical-pointtemperature. The temperature of the generated cryogen is lowered atblock 314 through heat exchange with a substance having a lowertemperature. In some instances, this may conveniently be performed byusing heat exchange with an ambient-pressure liquid state of thecryogen, although the heat exchange may be performed under otherconditions in different embodiments. For instance, a different cryogenmight be used in some embodiments, such as by providing heat exchangewith liquid nitrogen when the working fluid is argon. Also, in otheralternative embodiments, heat exchange may be performed with a cryogenthat is at a pressure that differs from ambient pressure, such as byproviding the cryogen at lower pressure to create a colder ambient.

The further cooled cryogen is provided at block 318 to acryogenic-application device, which may be used for a coolingapplication at block 322. The cooling application may comprise chillingand/or freezing, depending on whether an object is frozen with thecooling application. The temperature of the cryogen is increased as aresult of the cryogen application, and the heated cryogen is flowed to acontrol console at block 326. While there may be some variation, thecryogen pressure is generally maintained greater than the critical-pointpressure throughout blocks 310-326; the principal change inthermodynamic properties of the cryogen at these stages is itstemperature. At block 330, the pressure of the heated cryogen is thenallowed to drop to ambient pressure so that the cryogen may be vented,or recycled, at block 334. In other embodiments, the remainingpressurized cryogen at block 326 may also return along a path to block310 to recycle rather than vent the cryogen at ambient pressure.

There are a variety of different designs that may be used for thecryogen generator 246 in providing cryogen at a pressure that exceedsthe critical-point pressure, or meets the near-critical flow criteria,to provide substantially uninterrupted cryogen flow at a pressure andtemperature near its critical point. In describing examples of suchdesigns, nitrogen is again discussed for purposes of illustration, itbeing understood that alternative cryogens may be used in variousalternative embodiments. FIG. 4 provides a schematic illustration of astructure that may be used in one embodiment for the cryogen generator.A thermally insulated tank 416 has an inlet valve 408 that may be openedto fill the tank 416 with ambient liquid cryogen. A resistive heatingelement 420 is located within the tank 416, such as in a bottom sectionof the tank 416, and is used to heat the cryogen when the inlet valve isclosed. Heat is applied until the desired operating point is achieved,i.e. at a pressure that exceeds the near-critical flow criteria. A crackvalve 404 is attached to an outlet of the tank 416 and set to open atthe desired pressure. In one embodiment that uses nitrogen as a cryogen,for instance, the crack valve 404 is set to open at a pressure of about33.9 bar, about 1 bar greater than the critical-point pressure. Once thecrack valve 404 opens, a flow of cryogen is supplied to the system asdescribed in connection with FIGS. 2A and 2B above.

A burst disk 412 may also be provided consistent with safe engineeringpractices to accommodate the high cryogen pressures that may begenerated. The extent of safety components may also depend in part onwhat cryogen is to be used since they have different critical points. Insome instances, a greater number of burst disks and/or check valves maybe installed to relieve pressures before they reach design limits of thetank 416 in the event that runaway processes develop.

During typical operation of the cryogen generator, an electronicfeedback controller maintains current through the resistive heater 420to a level sufficient to produce a desired flow rate of high-pressurecryogen into the system. The actual flow of the cryogen out of thesystem may be controlled by a mechanical flow controller 208 at the endof the flow path as indicated in connection with FIG. 2A. The amount ofheat energy needed to reach the desired cryogen pressures is typicallyconstant once the inlet valve 408 has been closed. The power dissipatedin the resistive heater 420 may then be adjusted to keep positivecontrol on the mechanical flow controller 208. In an alternativeembodiment, the mechanical flow controller 208 is replaced with theheater controller for the cryogen generator. In such an embodiment, oncethe crack valve 404 opens and high-pressure cryogen is delivered to therest of the system, the feedback controller continuously adjusts thecurrent through the resistive heater to maintain a desired rate of flowof gaseous cryogen out of the system. The feedback controller may thuscomprise a computational element to which the heater current supply andflow controller are interfaced.

In another embodiment, a plurality of cryogen generators may be used toprovide increased flow for specific applications. Such an embodiment isillustrated in FIG. 5 for an embodiment that uses two cryogen generators512, although it is evident that a greater number may be used in stillother embodiments. The plurality of cryogen generators 512 are mountedwithin an ambient-pressure cryogen Dewar 502 that contains a volume ofambient-pressure cryogen 516. Near-critical cryogen generated with thecryogen generators 512 is provided to a heat exchanger 508 that coolsthe cryogen in the same manner as described with respect to the heatexchanger 242 of FIG. 2A. A crack valve 504 associated with each of thecryogen generators 512 permits the high-pressure sub-cooled (i.e. cooledbelow the critical temperature) cryogen to be provided tocryogen-application devices along tube 420.

In a specific embodiment, each of the cryogen generators has a generallycylindrical shape with an internal diameter of about 30 cm and aninternal height of about 1.5 cm to provide an internal volume of aboutone liter. The cryogen generators may conveniently be stacked, with eachcryogen generator having its own independent insulating jacket andinternal heater as described in connection with FIG. 4. A coil of tubingmay be wrapped around the outer diameter of the stacked cryogengenerators, with the output flow of high-pressure cryogen from eachcryogen generator passing through a respective check valve beforeentering the inlet side of the coiled tubing heat exchanger. An outletfrom the coil heat exchanger may advantageously be vacuum jacketed orotherwise insulated to avoid heating of the high-pressure cryogen as itflows towards the object being cooled. Such a stack of cryogengenerators and the outer-coil heat exchanger may be mounted towards thebottom of a liquid-cryogen Dewar, such as a standard Dewar that holdsabout 40 liters of liquid N₂ when full. This Dewar may also be equippedwith a convenient mechanism for filling the Dewar with liquid cryogenand for venting boil-off from the Dewar. In some instances, the liquidcryogen is maintained at or near ambient pressure, but may alternativelybe provided at a different pressure. For instance, the liquid cryogenmay be provided at a lower pressure to create a colder ambientliquid-cryogen bath temperature. In the case of liquid N₂, for example,the pressure may be dropped to about 98 torr to provide the cryogen atthe liquid-N₂ slush temperature of about 63 K. While such an embodimenthas the advantage of providing even lower temperatures, there may beadditional engineering complexities in operating the liquid-cryogenDewar below ambient pressure.

Operation of the multiple-cryogen-generator embodiments mayadvantageously be configured to provide a substantially continuoussupply of high-pressure cryogen to the cryogenic device. The ambientliquid-cryogen 516 is used as a supply for a depleted cryogen generator512, with the depleted cryogen generator 512 being refilled as anotherof the cryogen generators 512 is used to supply high-pressure ornear-critical cryogen. Thus, the example in FIG. 5 with two cryogengenerators is shown in an operational state where the first of thecryogen generators 512-1 has been depleted and is being refilled withambient liquid cryogen 516 by opening its inlet valve to provide flow520. At the same time, the second cryogen generator 512-2 has a volumeof liquid cryogen that is being heated as described so that cryogen isbeing delivered as near-critical cryogen through its outlet crack valve504. When the second cryogen generator 512-2 empties, the fill valve ofthe first cryogen generator 512-1 will be closed and its heater broughtto full power to bring it to the point where it provides near-criticalcryogen through its check valve. The inlet valve of the second cryogengenerator 512-2 is opened so that it may engage in a refill process, thetwo cryogen generators 512 thereby having exchanged roles from what isdepicted in FIG. 5.

The two cryogen generators 512 operate out of phase in this way untilthe entire Dewar 502 of ambient liquid cryogen is depleted, providing asubstantially continuous flow of near-critical cryogen to the cryogenicapplication devices until that time. The system is thus advantageouslyscalable to meet almost any intended application. For example, for anapplication defined by a total cooling time and a rate at which cryogenis consumed by providing a Dewar of appropriate size to accommodate theapplication. As will be noted later, the cooling capacity ofnear-critical liquid N₂ allows efficient consumption of cryogen formaximal operation times and scaling of near-critical cryogen generatorsto total freeze time requirements dictated by specific applicationneeds. For instance, the inventors have calculated that medicalcryogenic freezing applications may use near-critical cryoprobes thatconsume about two liters of ambient liquid N₂ per instrument per hour.

Cryogen consumption for multiple-probe use provides one demonstration ofthe relative efficiency of near-critical liquid-N₂ systems compared toJT-base units with their large associated tanks, leading to much smallerfunctional size of the whole system configuration. Specifically, testdata collected by the inventors suggests that a 1.6-mm near-criticalliquid-N₂ 1.6-mm cryoprobe generates iceballs up to 4 cm in diameterwithin gelatin phantoms, similar to 2.4-mm JT-based current argoncryoprobes. Based on tests performed by the inventors, it is believedthat 10 near-critical liquid-N₂ cryoprobes may produce a freeze volumecomparable to no less than six JT-based argon cryoprobes. This allows aclear demonstration of appropriate system contents for currentlyaccepted prostate use age of up to three cases per day. Current 2.4-mmcryoprobes utilize up to 40 PSI/min., thereby yielding no more than 80minutes of total freeze time for a single probe from a single full argontank (i.e. no more than 3200 PSI of usable pressure within a full6000-PSI argon tank). Therefore, at least two of these argon tanks (160minutes maximum) are generally used for each prostate case, whichusually requires at least six 2.4-mm cryoprobes operating for twoseparate freeze cycles, the first usually 15 minutes and the secondusually 10 minutes (i.e. (6×15)+(6×10)=150 probe-minutes total freezetime). In addition, one helium tank containing 2000 PSI is also used foreach prostate case and the tank is of similar size and weight to theargon tanks. Combining the 80 kg (175 lbs), 0.38-m³ Endocare system box(i.e. dimensions=125×48×64 cm³) with the nine tanks (three for eachcase) weighing 182 kg each (1638 kg total) and displacing 0.062 m³ each(π×12 cm²×137 cm×9=0.56 m³ total) amounts to a total system weight of1718 kp (i.e. 3780 lbs or 1.9 tons) within a 0.94-m³ volume.

For the near-critical liquid-N₂ system, even ten cryoprobes used inparallel for 75 minutes (i.e. 25 minutes for each case) may thus beaccommodated by a 25-liter Dewar (10 probes×2 L/(probe hour)×1.25hours). Such a 25-liter Dewar may measure approximately 30 cm indiameter and 1.0 meter in height (i.e. 0.07 m³ outer volume containingan insulated inner reservoir of 20-cm diameter and 0.75 meters inheight, holding the 25-liter volume). This full Dewar may weight up to40 kg (i.e. 20-kg Dewar weight+20 kg for 25 liters of liquid N₂ at 0.81g/cm³) and may be at least partially contained within the relativelyempty Endocare box dimensions. A self-contained near-critical systemprovided by embodiments of the invention may thus weigh less than 120 kp(i.e. 80-kg box+the 40-kg full Dewar) and occupy a space of less than0.40 m³ (i.e. 03.7+0.07/2 m³). Therefore, the following scaling chartprovides a comparison of system performance in relation to practicalclinical footprint within a surgical or radiological suite. For ahandheld embodiment measuring approximately 25 cm in length, 50 cm indiameter, and containing 100 mL of liquid N₂ that can be run at variablerates for dermatological or limited interstitial freeze times:

Total Freeze Weight Volume Type Time (min) (kg) (m³) Case Tanks JT-Argon25 626 0.56 1 prostate 2 Ar + 1 He JT-Argon 75 1718 0.94 3 prostate 2Ar + 1 He NC-LN₂ 75 120 0.40 3 prostate none NC-LN₂ 5-20 1 0.0005 1US-guided single probe

In summary, a near-critical liquid-nitrogen system may perform a fullclinical day's caseload with less than half the clinical spacerequirement and an order of magnitude lower total weight that needs tobe move around by technicians in setup, compared to current JT-basedsystems. Quantification of lower production costs is complicated by theexistence of regional cost differences, but it is noted that currentlycosts for JT-based systems include an average un-reimbursable cost ofabout $200/case for compressed gases. The quantification of weight andspace requirements also do not account for work time and safety impactsof hospital personnel having to move, connect, and secure high-pressuretanks.

Some embodiments of the invention are especially suitable for low-volumeshort-duration cryogenic applications and are provided in the form of aself-contained handheld instrument, an example of which is shown withthe photograph of FIG. 6. The integrated handheld instrument isespecially suitable for use in applications involving a relatively briefcryogenic cooling, such as dermatology and interstitial low-volumefreeze applications (e.g., treatment of breast fibroadenomas,development of cryo-immunotherapy). The structure of such an instrumentis substantially as described in connection with FIG. 2A, with thecomponents provided as a small self-contained unit. In particular, arelatively small cryogen generator 604 is connected in series with asmall ambient liquid-cryogen tank 608, and a mounted cryogenic device612. In the example shown in FIG. 6, the cryogenic device is acryosurgical device that is permanently mounted to the instrument,although other types of cryogenic devices may be used in differentembodiments. The self-contained handheld instrument may be provided as adisposable single-use instrument or may be rechargeable with liquidcryogen in different embodiments. The cryogen generator 604 and ambientliquid-cryogen tank 608 are vacuum jacketed or otherwise thermallyinsulated from their surrounding environment and from each other. Forconvenience of display, the instrument shown in FIG. 6 was photographedwith an outer tube that holds the cryogen generator 604 andliquid-cryogen tank 608 under vacuum removed.

To be used, a switch is provided that allows an operator to control asmall heater in the cryogen generator. The activation of the heaterresults in a flow of near-critical cryogen through set flow impedancesthat may be customized for a particular cooling task as described above.The flow of near-critical cryogen may continue until a reservoir of suchcryogen within the instrument is expended, after which the instrumentmay be disposed of or recharged for future use. In some embodiments,such as for the cooling of sensitive receiver electronics, the handheldunit may interface with the object being cooled through a self-sealinginterconnect port. This permits the object to be cooled to bedisconnected from the disposable or rechargeable instrument betweenuses.

The handheld-instrument embodiments may be considered to be part of thecontinuum of scalability permitted by the invention. In particular,there is not only the option of providing sufficient near-critical orhigh-pressure cryogen for high-volume clinical or other uses, but alsofor short-duration low-volume uses. Over the full range of thiscontinuum, operation is possible with very small cryogenic-device sizes,i.e. less than 1 mm, because there is no barrier presented by thephenomenon of vapor lock. For example, the ability to operate with smalldevice sizes enables a realistic arrangement in which small rechargeableor disposable liquid-cryogen cartridges are provided as a supply,removing the need for large, inconvenient cryogenic systems. Forinstance, in the context of a medical application such as in a clinicalsetting for nerve ablation, or pain treatment, a small desktop Dewar ofliquid N₂ may be used to provide liquid N₂ for refilling multiplecartridges as needed for nerve ablation. For a typical volume in such aclinical setting, the desktop Dewar would require recharging perhapsonce a week to provide enough liquid for refilling the cartridges foruse that week. Similar benefits may be realized with embodiments of theinvention in industrial settings, such as where short-term cooling isprovided by using disposable cartridges as needed. A minor accommodationfor such applications would provide appropriate venting precautions forthe tiny amount of boil-off that is likely to occur, even withwell-insulated and/or pressurized cartridges. Embodiments of theinvention thus enable an enhanced scope of cryogenic cooling options fornumerous types of applications.

A further embodiment uses the generation of near-critical cryogen fordirect spraying for both medical (e.g., dermatology, and as anintraoperative surgical assist tool during any tumor resection) andnon-medical applications. Current LN₂ dispensers for dermatology dependupon the spontaneous boiling of liquid N₂ through a pinhole nozzle,producing slow and relatively inaccurate dispensing of the cryogen, i.e.it effectively splatters out through the tiny hole. In addition, theinability to precisely control the freeze margins results in widerdestruction of skin lesions with associated collateral damage ofadjacent normal skin tissue. The sputtered delivery also makes liquid N₂application close to crucial structures, such as small fatty depositssurround the eyes, more dangerous and generally avoided. FIG. 7A shows abattery-powered canister 702 with a small battery 704 placed along aback of the canister 702. The battery 704 drives the electric currentalong wires 706 to a coiled configuration 708 in the bottom of thecanister. The canister may be filled with liquid nitrogen 710 and thelid 712 replaced. When the lid 712 makes a tight seal, a low powercurrent flows to the coil 708 to generate critical-N₂ pressures(e.g., >600 PSI). The system is now in a charged state and ready foruse, venting excess pressure (e.g. >700 PSI) through a calibrated valve714. When a trigger 716 is pulled, a variable power control 718activates increased electrical power to cause accelerated critical-N₂production during spraying. Pulling the trigger 716 also activates themanual release of a special pressure valve 720 that is configured toclose automatically if the pressure drops below 600 PSI. In this manner,only critical N₂ is released through the tip mechanism, avoiding vaporlock and/or sputtering. In order to achieve a precise but powerfulspray, a pencil-point tip 722 may be provided to permit fine control bythe physician.

Direct skin, or intraoperative, applications of critical N₂ may befurther controlled by application of a vented cone tip 724. For example,this tip 724 may be provided as a disposable, single-use item (e.g.polyurethane) that gets quickly connected to the nozzle in place of thepencil-point tip 722. An open end of the cone may be provided in severalvariable sizes to accommodate a range from very small to larger lesions(e.g., 3, 5, 7, 9, 11, 13, 15, etc. mm). In operation, the cone tip isplaced over a lesion, accommodating both exophytic (e.g. skintags), aswell as flaw lesions (e.g. age spots). The critical N₂ travels down acentral delivery tube, while the reflected spray gets vented out higheralong sides of the cone. In this manner, only the skin/organ exposedbeneath the cone tip containing the lesion gets treated and the ventedgas stays away from the treatment site, further preserving collateraltissue.

FIG. 7B illustrates that in still a further embodiment a vented needle730 may be placed as the dispenser tip instead of pencil-point tip 722or the vented cone 724. Such needles may be approximately 1 mm in outerdiameter. Critical N₂ may travel down a central shaft 732 to a chamber736 within the closed tip. For dermatological/intraoperative uses, thereturning nitrogen 734 may be vented near the needle hub, or along anassociated connecting tube. Such a needle may not require an insulatedshaft since it may be directly placed into a skin, or intraoperative,tumor 738, potentially freezing along its entire exposed length.

Thermometry may also play a role in the treatment of skin, orintraoperative, tumors, helping to ensure thorough ablation of commonmalignant lesions. Basal cell carcinoma has been noted to requireextremely low temperatures for complete necrosis, approaching −160° C.Therefore, a needle thermometry array may be placed directly beneath atumor using either palpable or ultrasound guidance. Freezing may thensafely progress until uniform thermometry readings of −160° C. arereached by either concentrated direct spray technique, interstitialneedle placement, or their combination. Highly controlled tumor ablationof skin masses may also use a tabletop, multi-probe unit that allowscomputer control of the freezing process. For example, thermometryarrays 742 placed beneath the tumor would allow direct feedback tointerstitial cryoneedles 740 placed within the tumor, shutting them offas soon as appropriate cytotoxic low temperatures are reached. Thephysician using a separately controlled, direct-spray technique may alsodirectly treat more superficial aspects of the tumor.

Embodiments of the invention provide increased cooling power whencompared with simple-flow cryogen cooling or with Joule-Thomson cooling,with one consequence being that the need for multiple high-pressuretanks of cryogen is avoided even without recycling processes. Acomparison is made in FIG. 8 of the cooling power per mole of cryogenfor the three different cooling systems. The top curve corresponds tothe cooling cycle described herein in connection with FIG. 2B using N₂as the cryogen, while the bottom two points identify the cooling powerfor Joule-Thomson processes that use argon and nitrogen as cryogens. TheJoule-Thomson results represent maximum values for those processesbecause they were determined for perfect counter-flow heat exchange;this heat exchange becomes very inefficient as the probe diameter isreduced.

The presented results note that vapor lock of liquid N₂ may occur atlower pressures, but is avoided in the circled region 804 when theprocess meets the near-critical conditions for pressures near thecritical-point pressure for N₂ of 33.94 bar. As previously noted, vaporlock may be avoided at near-critical flow conditions, although theefficiency of the process is improved when the pressure is near thecritical-point pressure. The results illustrate that cooling cyclesprovided according to embodiments of the invention are more than fivetimes as efficient as idealized Joule-Thomson cycles. Since theefficiency of embodiments that use pressures above the critical-pointpressure is not substantially affected by changes in probe size, thecooling power per gram is often more than ten times greater than thecooling power for Joule-Thomson cycles. This greater efficiency ismanifested by the use of substantially less, i.e. ⅕th- 1/10 th, of theexhaust gas flow, making the process much quieter, less disruptive, andwithout the need for bulky multiple-tank replacements.

Thus, having described several embodiments, it will be recognized bythose of skill in the art that various modifications, alternativeconstructions, and equivalents may be used without departing from thespirit of the invention. Accordingly, the above description should notbe taken as limiting the scope of the invention, which is defined in thefollowing claims.

1. A method for cooling the surface of a cryoprobe using a cryogenhaving a critical point defined by a critical-point pressure and acritical-point temperature, the method comprising: raising a pressure ofcryogen above its critical-point pressure, said cryogen comprising N₂;and thereafter, placing the cryogen in thermal communication with thesurface of a cryoprobe to increase a temperature of the cryogen along athermodynamic path that maintains the pressure greater than thecritical-point pressure for the duration that the cryogen and saidsurface are in thermal communication.
 2. The method recited in claim 1wherein the thermodynamic path increases the temperature of the cryogento an ambient temperature.
 3. The method recited in claim 2 furthercomprising reducing the pressure of the cryogen to an ambient pressureafter removing the cryogen from thermal communication with said surface.4. The method recited in claim 1 further comprising, after raising thepressure of the cryogen above its critical-point pressure and prior toplacing the cryogen in thermal communication with said surface, reducingthe temperature of the cryogen without decreasing the pressure of thecryogen below its critical-point pressure.
 5. The method recited inclaim 4 wherein reducing the temperature of the cryogen comprisesplacing the cryogen in thermal communication with a second liquidcryogen having a temperature lower than the temperature of the cryogen.6. The method recited in claim 5 wherein the cryogen and second cryogenare chemically identical.
 7. The method recited in claim 5 wherein thesecond liquid cryogen is substantially at ambient pressure.
 8. Themethod recited in claim 1 wherein raising the pressure of the cryogenabove its critical-point pressure comprises: placing the cryogen withina thermally insulated tank; and applying heat within the thermallyinsulated tank at least until a predetermined pressure within thethermally insulated tank is reached.
 9. A system for cooling the surfaceof a cryoprobe using a cryogen having a critical point defined by acritical-point pressure and a critical-point temperature, the systemcomprising: means for raising a pressure of cryogen above a pressurevalue determined to provide the cryogen at a reduced molar volume thatprevents vapor lock, said cryogen comprising N₂; and means forthereafter placing the cryogen in thermal communication with the surfaceof a cryoprobe to increase a temperature of the cryogen along athermodynamic path that maintains a pressure greater than determinedpressure value for the duration that the cryogen and said surface are inthermal communication.
 10. The system recited in claim 9 furthercomprising means for reducing the pressure of the cryogen to an ambientpressure after removing the cryogen from thermal communication with saidsurface.
 11. The system recited in claim 9 further comprising means forreducing the temperature of the cryogen without decreasing the pressureof the cryogen below its critical-point pressure after raising thepressure of the cryogen above its critical-point temperature and priorto placing the cryogen in thermal contact with said surface.
 12. Thesystem recited in claim 11 wherein the means for reducing thetemperature of the cryogen comprises means for placing the cryogen inthermal communication with a second liquid cryogen having a temperaturelower than the temperature of the cryogen.
 13. The system recited inclaim 9 wherein the means for raising the pressure of the cryogencomprises means for applying heat to the cryogen within a thermallyinsulated holding means until a predetermined pressure within thethermally insulated holding means is reached.